Subsidiary Communication Authorization or SCA receivers refer to a process of multiplexing an additional program on an FM broadcast station transmission in such a manner that this additional program cannot be heard by the casual listener, but rather is demodulated through the utilization of a specialized receiver so that it can be heard by a special limited group of recipients.
The original and most common use for such transmissions is background music which is transmitted as a subcarrier on an FM broadcast. While background music has been in existence for a number of years, the technology has evolved into numerous other forms of narrowcasting, as opposed to broadcasting. It will be appreciated that a broadcast is intended for general listenership, whereas a narrowcast is intended for a limited audience who pays for the service.
Typically in order to generate the SCA format a specialized generator is required at the transmitter site which is called a multiplexer generator and typically imposes a low amplitude supersonic tone which is frequency modulated by the program to be transmitted. Most commonly the frequency utilized for the supersonic tone is 67 kHz, although in the past 41 kHz has been used. However since 41 kHz signals have been preempted by stereo broadcasting, other frequencies are now utilized for SCA systems. In addition to 67 kHz, 92 kHz is often used. FCC regulations now permit the use of any frequency band within 53 kHz and 99 kHz.
With respect to the fidelity of such SCA broadcasts, the audio bandwidth does not typically exceed 5 kHz, with the spectrum occupied being typically about 15 kHz.